Renewable energy power systems, such as wind energy power systems and solar energy power systems, can include energy storage systems, which are often coupled to a DC bus of a power converter. For example, wind power systems, such as wind driven doubly-fed induction generator (DFIG) systems or full power conversion systems, can include a power converter with an AC-DC-AC topology and an energy storage system coupled to the DC bus in the power converter.
In a typical configuration, an energy storage system can be configured to store energy generated by the renewable energy power system or provide stored energy from the energy storage system to an electrical grid. However, in an energy storage system coupled to a DC bus in a power converter, large cabinet filters are often needed to filter out an AC component superimposed on a DC component (i.e., ripple and common mode noise with differential mode noise) before energy is provided to one or more energy storage devices in the energy storage system. These filters can be very expensive, complex, and add failure modes, which can reduce reliability, availability, and require resources for maintenance. Further, the cost of these filters can increase as the size of the filter increases. Further, in such a configuration, the one or more energy storage devices in the energy storage system, such as one or more batteries, may not be electrically isolated from the power converter in the renewable energy system, and thus can be susceptible to damage from fault currents and a cause of serious concern for safety due to the floating configuration.
Moreover, in a typical energy storage system coupled to a DC bus of a power converter, energy stored in the energy storage system may not be available during various operating conditions, such as when an electrical grid is experiencing a fault condition requiring isolation of the renewable energy power system as power from the grid may be needed to power a control device configured to control operation of the energy storage system. Further, during excessive wind conditions, grid power may be unavailable to power a yaw system in order to yaw a wind turbine out of the wind to reduce stress on the wind turbine, and aerodynamic braking may be unable to sufficiently slow or stop the wind turbine.